FIG. 1 shows the general structure of such a matrix, where CL denotes a liquid crystal cell and Q denotes the transistor associated with this cell, the whole assembly of the cell and of the transistor forming the pixel. The counter-electrode of the cell is denoted by CE, and the electrode of the pixel is denoted by Ep. The row control conductors are denoted by L1 to Ln for a matrix with n rows. The column conductors are C1 to Cm for a matrix with m columns. A row decoder DEC addresses the various rows successively. During the addressing of a row, a digital-analog conversion circuit DAC applies a set of analog voltages to the column conductors representing the image to be displayed by this row. The conversion circuit establishes these analog voltages starting from a digital signal. A sequencing circuit SEQ ensures the synchronized operation of the row decoder and of the conversion circuit DAC.
In most liquid crystal displays, the liquid crystal is placed between a plane containing the counter-electrode and a plane containing the pixel electrode and the control transistor. The counter-electrode is common to all the pixels of the matrix and covers the whole matrix. The pixel electrodes each cover a large part of the pixel. The pixel electrode and the counter-electrode are transparent (made of indium-tin oxide ITO) in the case of a transmissive display; one of the two is reflecting in the case of a reflective display. The electric field applied is perpendicular to the plane of the display. At rest, in other words in the absence of this field, the molecules tend to be oriented in the plane (or respectively perpendicularly to this plane, depending on the type of molecule). In the presence of an electric field, their orientation re-aligns so as to become perpendicular (respectively parallel) to the plane.
The use of indium-tin oxide involves costly fabrication operations; the material is costly; moreover, this material has a high optical index (almost equal to 2) and causes undesirable reflections and hence losses of light; lastly, it is not perfectly transparent, notably in the green wavelengths where the eye is particularly sensitive; its thickness cannot be reduced too much in order to make it more transparent because it needs to remain sufficiently conducting in order to establish uniformly the desired electric field.
In order to avoid the drawbacks of indium-tin oxide, displays have already been provided in which the applied electric field is in the plane of the display rather than perpendicular to this plane. These displays are referred to as “In-Plane Switching Displays”. At rest, the molecules are for example oriented in a direction Ox of this plane; when an electric field is applied in a direction Oy different from Ox but still in the same plane, the molecules of the liquid crystal tend to rotate in the plane and to orient themselves in the direction Oy. The polarization of the light which results from this rotation of the molecules is modified. Upstream and downstream polarizers allow the intensity of light which passes through the display to be modulated in proportion to the variations in orientation of the molecules.
In these displays, the electric field is applied between two electrodes situated in the plane of the display. The counter-electrodes are placed along the edges of the pixel. If the dimensions of the pixels are small (a few micrometers on a side), a single electrode placed in the middle of the gap between the counter-electrodes suffices for establishing an electric field capable of making the molecules rotate with a reasonably low voltage (a few volts). For pixels with larger dimensions, pixel electrodes and counter-electrodes may be interleaved in order to establish this field. The electrodes and counter-electrodes do not need to be transparent because they only occupy a fraction of the surface area of the pixel. They can be formed prior to the installation of the liquid crystal using a non-transparent metal conventionally used in microelectronics technologies (aluminum, copper, etc.).
In-plane switching displays are furthermore better from the point of view of the angle under which they can be viewed. This comes from the fact that the ellipsoid of the indices of the molecules used is oriented with its large axis in the plane both at rest and when an electric field is applied; the variations in optical delay of the light as a function of the viewing angle are smaller than when the molecules have their index ellipsoid oriented vertically (as is the case for conventional switching displays using an electric field perpendicular to the plane).
However, one drawback of in-plane switching displays is the fact that the return forces exerted on the molecules to return them to their rest position after removal of the electric field are not very high. The reason for this is that only the twist forces act but not the splay forces or the bend forces which are exerted in the case of rotations out of the plane.
This results in a limitation in the speed with which it is possible to go from one image to another.
This flaw is particularly sensitive if the display has to operate in color sequential mode. It is recalled here that this mode consists in successively displaying on the same liquid crystal matrix three images corresponding to the three primary colors to be displayed and in illuminating the matrix with a source of one primary color in strict synchronism with the display of the image corresponding to this color. Color sequential mode is advantageous because it avoids having three different matrices for displaying the three colors or having a mosaic of colored filters on a single matrix. However, on the downside, in order to display color images at a given rate (24 images per second for example), the matrix has to display partial images at a rate that is three times higher (72 images per second).